An out-of-control grass fire north of Goulburn has prompted warnings for any nearby residents still in the area to seek shelter.

Fire crews and a large air tanker are battling the blaze that reached emergency level on Thursday afternoon.

The fire is threatening properties 35km north east of Crookwell as it heads in an easterly direction towards Jerrong Road, having burnt more than 250 hectares.

"If you are in the area of Curraweela between Taralga Road and Jerrong Road, including Old Station Creek Road, it is now too late to leave," the NSW Rural Fire Service said.

NSW residents and firefighters are bracing for more unseasonably hot conditions as summer's hangover continues in parts of the state.

Temperatures are expected to reach the high 30s in some regions on Thursday and Friday, with inland areas tipped to get into the mid 40s, sparking widespread fire warnings.

Firefighters managed to control a major blaze at Alpha Rd in the Central Tablelands that burnt more than 18,000 hectares. They warn the hotter-than-averages temperatures could lead to more flare-ups.

The Bureau of Meteorology issued extreme fire warnings for the Southern and Central Ranges, with much of the rest of the state considered a high fire risk.

"This is fairly transient weather at this time of the year," BOM senior meteorologist Olenka Duma said.

"We start to see cold fronts making their way further northwards in the autumn.

"With those cold fronts we often see warmer temperatures, which are still occurring in northern and central parts of Australia, dragged across NSW."

Temperatures in Sydney are expected to exceed 35C on Thursday, before cooling slightly with maximums of 30C and 31C over the next three days.

The BOM on Tuesday placed a watch warning on its El Nino outlook, meaning there is a 50 per cent chance the weather driver could hit Australia this year, bringing hotter and drier conditions.

Wetter conditions from three consecutive La Ninas have contributed to more grass growth and vegetation, potential fuel for bushfires.

With warmer than average temperatures and drier conditions expected to stretch into winter, the fire risk will linger later than usual.

Total fire bans are in place for the Central and Southern Ranges, Southern Slopes and Lower Central West Plains.